Peak current definition

the switching transistor, Q1, is in full saturation with

maximum output power conditions, assuming

worse-case input voltage and transistor gain under all

operating temperature extremes.

determined by:

R

V

I

SENSE

SENSE

PK

=

Sense Resistor

A low value sense resistor is required to set the peak

current. Power in this resistor is negligible due to the

the page there is a table of recommended sense

resistors.

Output power calculation

By making the above assumptions for inductance and

peak current the output power can be determined by:

(Watts)

where

I

I

2

(T

T

)

(T

T

)

AV

PK

ON

DIS

ON

OFF

=×

+

+

and

T

IL

V

ON

PK

IN

=

×

and

T

IL

(V

V )

DIS

PK

LED

IN

=

×

−

and

and

= efficiency e.g. 100% = 1

Operating frequency can be derived by:

F

1

TT

ON

OFF

=

+

Programming LED current

Once the required output power is determined, the LED

current can be programmed by adding a single resistor

in the LED chain. The resistor value is determined by

the following:

I

V

R

LED

FB

LED

=

RLED selection table

Shutdown Control

The ZXSC400 offers a shutdown mode that consumes

a standby current of less than 5 A. When the voltage at

the ZXSC400 is enabled and the driver is in normal

operation. When the voltage at the STDN pin is 0.7V or

lower, the ZXSC400 is disabled and the driver is in

shutdown mode. The SHDN input is a high impedance

current source of 1 A typ. The driving device can be an

open collector or an open drain or a logic output with a

“High“ voltage of 5V max. The device shutdown

current depends of the supply voltage, see typical

characteristics graph

ZXSC400

ISSUE 1 - JANUARY 2003

6

Manufacture

Series

R

) Range

Size

Tolerance

URL

Cyntec

RL1220

0.022 - 10

0805

±5%

www.cyntec.com

IRC

LR1206

0.010 - 1.0

1206

±5%

www.irctt.com

I

LED

R

LED

40mA

7.5

30mA

10

20mA

15

15mA

20

10mA

30